1. Field of the Invention
The present invention relates to the field of plastic coating of wood.
2. Prior Art
U.S. Pat. No. 4,181,764 describes a process for the plastic coating of wood members such as fence rails and posts which results in a very attractive, relatively maintenance free and long-life decorative fencing. In accordance with the process, the wood members are introduced along their longitudinal axis into a crosshead die mounted on a plastic extrusion machine, the same extruding a thermoplastic resin onto the wood, forming a protective coating thereon as the wood passes through the crosshead die. In general, the rails or fence posts, depending upon the size of the die being used, are passed therethrough end to end at a rate slightly faster than the rate of extrusion so that the extruded coating is somewhat pulled longitudinally while still being warm. This in turn causes the extruded coating to want to decrease in transverse dimension. If the wood members being coated have sharp corners thereon, this puts considerable tension on the coating, tending to draw the same tightly against the sharp corners, resulting in the plastic coating drawing thin thereover or even splitting the coating along the edge.
This problem can be avoided by the removal of the material constituting the sharp edge either by hand, or by an appropriate powered cutting tool. Hand removal is suitable for test purposes, though it is way too slow and inefficient for production purposes. Hand removal using an appropriate power tool is much better, though still is much more time consuming and expensive than desired.
Since the wood members are feed end to end through the crosshead die, one can readily feed the members end to end through an appropriate powered cutting machine prior to the crosshead die, to automatically remove the corners just before the members proceed into the crosshead die for coating. Because the rails as well as the posts are well controlled in cross-sectional dimensions so that the respective crosshead die of fixed dimensions will provide a substantially uniform coating on all rails or posts, such a cutting machine, in theory, can readily provide the desired result. In practice however, the wood members, particularly the rails, frequently have some twist end to end. Thus, accurately aligning and guiding the wood members through the cutters from a location other than the longitudinal position of the cutters can result in some skew of the cross-section of the wood member at the cutters, resulting in an excessive cut on one or more corners and an inadequate cut on other corners, the extent of which may vary along the length of the wood member. In that regard, while neither posts nor rails having any substantial bend or curve therein may be used and still achieve the ultimate desired aesthetic appearance, particularly the rails may have substantial twist from end to end and still be useable, as any such twist will be taken out when the ends of the rail are fastened between two parallel posts on installation.
Inadequate removal of the sharp corner, of course, will leave a sufficient corner to still cause thinning of the plastic coating at the corner region. Excessive breaking of the corner, on the other hand, will result in the plastic coating being particularly thick in the corner regions. This too is undesireable, as the thicker region will not stretch as much as the thinner sections in response to the lateral tension which develops prior to and as the freshly applied coating cools. Also, the thicker plastic will tend to hold the heat, making the coating immediately adjacent thereto slower to cool and thus more prone to stretching in response to the lateral tension. The net result can be an irregularity in thickness in the coating in that region, giving a perceivable streak or depression in the coating surface in that region.
The ideal coating is a coating of uniform thickness over a wood member of relatively closely controlled dimensions and with rounded corners. It is to this end that the crosshead die is proportioned and that the plastic extrusion rate and feed rate of the wood therethrough is predetermined. Accordingly, a sharp corner on the wood will result in a very thin, if any, coating thereover at the time of extrusion, even before the lateral tension causes further thinning and/or separation of the coating at the corner. Excessive removal of a corner, on the other hand, may result in inadequate filling of the corner region, or even with adequate filling, may result in a waviness in the surface of the junction of the thick and thinner sections after cooling.